The purpose of this study is to determine if the combination of VELCADE and rituximab improves progression free survival relative to rituximab alone in patients with relapsed or refractory B-cell non-Hodgkin's lymphoma (B-NHL) who never received rituximab or who have previously responded to rituximab. This is an international study being conducted in the United States and in many countries around the world. A complete list of study locations is listed below.